Sacrificial layers of selected sacrificial materials are commonly used in fabrications of microstructures, such as microelectromechanical systems and semiconductor devices. A typical and pervasively used sacrificial material is amorphous silicon. Once the desired structures of the microstructure are formed, the sacrificial layers are removed by etching. The success of the etching process depends upon the selectivity of the etching process. Performance, uniformity and yield can all be improved with increases in the etch selectivity of the sacrificial layers.
More recently, the etching method using selected gas phase etchants has drawn great interest in fabricating microstructures due to its many advantages, such as high selectivity, less contamination and less process stiction as opposed to other possible etching methods, such as a wet etching techniques. In terms of the different ways of feeding the selected gas etchant into the etch chamber containing the microstructure to be etched, the current gas etching method has two major categories—continuous etchant feeding and one-time (Batch) etchant feeding. In a typical continuous etchant feeding process, the gas etchant continuously flows through the etch chamber until the sacrificial materials of the microstructure are exhausted by the chemical reaction inside the etch chamber. This etch process is unfavorable because the continuous flowing of the gas etchant etches the sacrificial layers too fast which makes the etching process difficult to control. Moreover, the continuous flow is inefficient in usage of etchant. In a typical one time etchant feeding process, the selected gas etchant is introduced into the etch chamber at one time and a chemical reaction occurs between the gas etchant and the sacrificial materials inside the etch chamber. This etch feeding technique improves the etchant usage efficiency and the possibility of precise control of the etching process. However, it also has disadvantages. For example, because the gas etchant and the sacrificial materials and the chemical reaction therebetween are confined in the etch chamber through out the etching process, the etching product (reaction product) will accumulate within the etch chamber. The accumulation may result the deposition of the etching products on the surface of the microstructure. At an extreme situation, the chemical reaction may be reversed, yielding re-deposition of the sacrificial material. In addition, because the amount of the etchant fed into the etching system at one time is fixed and the maximum amount of the sacrificial material that can be removed by the fixed amount of the etchant is limited for a given etching system, the maximum amount of the etchant fed into the etching at one time may not be enough to remove a larger amount of the sacrificial material. In an approach to solve this problem, additional amounts of the etchant are fed into the etching system in a discontinuous fashion. For example, in feeding an additional amount of the etchant, the etching system is pumped out and then provided with the additional amount of the etchant. During the pumping out process, the chemical reaction between the etchant and the sacrificial material, thus the etching process is stopped until the additional amount of the etchant is provided. This feeding process, however, may cause “etch front marks” and/or etching un-uniformities in the microstructures after etch. For example, when the first amount of the etchant fed at one time into the etching system is not enough to remove all sacrificial materials in the microstructure, the boundaries of the sacrificial material (the etch front) may create “marks” in the structures of the microstructure when the chemical reaction (etching process) is stopped due to the lack of the etchant. These “marks” may be permanent through out and even after the etching process.
Accordingly, a method and apparatus is desired for efficiently removing sacrificial layers in microstructures using selected gas phase etchant.